Low frequency plasma etching systems are well known. In such systems, a planar reactor is provided into which a gas mixture, such as Argon, carbon tetrafluoride, and fluoroform is injected at a low pressure. A pair of electrodes within the reactor receives power from a source of low frequency outside the reactor and the gas is ionized to form a plasma. A semiconductor wafer, such as silicon with a silicon dioxide film, is placed on one of the electrodes. The applied power to the electrodes and plasma formed within the reactor cause etching of the film in areas where no photoresist material is present.
One of the problems in many low frequency plasma etching systems is that there are a number of non-uniformity problems which are evidenced by some areas of the wafer etching faster than other areas. These non-uniformities are of various sorts. Some of them involve very gradual non-uniformities over the large areas of the wafers while others are very localized. Such localized patterns include small spots which tend to etch very fast. In other cases, small, narrow rings are formed all around the wafer at a certain radii where etching occurs as much as 10 or 20% faster than the rest of the wafer. Non-uniformities are visible to the eye when the wafer is removed from the reactor after being partially etched.
Non-uniformities, being visible, appear as colored interference patterns on a wafer with a silicon dioxide film. Some parts of the surface of the wafer become defective and may not yield working devices when the wafer is ultimately diced. In semiconductor device fabrication, sufficient over etch time is required to insure that the film is completely etched in the slowest area of the wafer. This means that in areas of faster etching, the substrate under the film being etched is subject to attack and possible damage by the plasma.